1. Field of the Invention
The present invention relates to a thin-film deposition system having a vacuum chamber containing a plasma generator.
2. Description of Related Art
A plasma generator converts gaseous molecules and evaporated particles into a high-density plasma in a thin-film deposition system, such as an ion plating system, for forming a thin film on a substrate and assisting in impinging the ions contained within the plasma onto the substrate.
FIG. 3 schematically shows one example of a thin-film deposition system incorporating a plasma generator. A crucible 3 holding an evaporable material 2 therein is mounted at the bottom of a vacuum chamber 1. An electron gun 4 for emitting an electron beam toward the evaporable material is also mounted at the bottom of the chamber 1.
A rotatable substrate dome 5 on which plural substrates are set is mounted near the top of the vacuum chamber 1. The dome is equipped with a heater 6.
A plasma generator 7 emits an electron beam to the space between the substrate dome 5 and the crucible 3.
The plasma generator 7 includes a cathode 8 made of thermionic tungsten filament or other material. The cathode 8 is connected with a heater power supply 9. An electric discharge chamber is formed inside a cylindrical case 10. The pressure inside the discharge chamber is made higher than the pressure inside the vacuum chamber 1 by argon gas introduced from a gas inlet port 11.
A first anode 12 is water-cooled and connected with a discharge power supply 13 via a resistor R1 having resistance R1. A second anode 14 is mounted to cover the surface of the first anode. A part of the first anode 12 is connected to a part of the second anode 14 to increase the thermal resistivity between them.
A shield body 15 is held to the case 10. An orifice permitting passage of the electron beam is formed at the front end of the shield body 15. A coil 16 consists of an electromagnet for producing a magnetic field parallel to the direction in which electrons are extracted. A plasma 17 created in the case 10 is focused toward the center axis of the case by the coil 16.
The cathode 8, first anode 12, second anode 14, case 10, resistor R1, heater power supply 9, and discharge power supply 13 together form a discharge circuit, which in turn is connected with the vacuum chamber 1 via a resistor R2 having resistance R2.
In the plasma generator 7, a given amount of argon gas is first introduced into the case 10 from the gas inlet port 11 to increase the pressure inside the case. The cathode 8 is heated to a temperature at which thermionic emission is possible by the heater power supply 9. Then, the coil 16 is energized with a given electrical current to induce plasma ignition and produce a magnetic field necessary to obtain a stable plasma.
Under this condition, if a given voltage, for example, of 100 V is applied between the cathode 8 and the anode assembly (12, 14) from the discharge power supply 13, an electric field 18 is produced over the orifice formed in the shield body 15. Thermoelectrons emitted from the cathode 8 are started to be accelerated toward the anode assembly (12, 14) by the electric field. The acceleration of the thermoelectrons causes repeated collisions of the thermoelectrons with the introduced argon gas, producing the plasma 17 inside the case 10.
The electrons produced inside the plasma 17 in this way are drawn into the vacuum chamber 1 by the electric field 18 while focused toward the center axis of the case by the magnetic field produced by the coil 16.
On the other hand, inside the vacuum chamber 1, an electron beam 19 from the electron gun 4 is directed at the evaporable material 2. The material is heated and evaporated. A process gas (e.g., oxygen gas) is introduced into the vacuum chamber 1 from a process gas inlet port 20.
The electrons extracted into the vacuum chamber 1 are made to collide against the process gas and particles of the evaporated material inside the vacuum chamber. The gas and particles are excited and ionized. Consequently, a plasma 22 is produced inside the vacuum chamber. The evaporated particles ionized within the plasma are drawn to the substrate set on the substrate dome 5 and adhered to the substrate. A film of the particles of the evaporated material is formed on the substrate.
The electrons extracted into the vacuum chamber 1 and the electrons within the plasma 22 flow into the wall of the vacuum chamber 1 and into the anodes 12, 14, maintaining a stable electric discharge.
Where an optical thin film is formed by the thin-film deposition system designed as described above, particles of evaporated, non-conductive dielectric materials that form the optical thin film adhere to the inner wall of the vacuum chamber 1, increasing the impedance. Therefore, most of the electrons extracted into the vacuum chamber 1 and the electrons within the plasma 22 are forced toward the anodes 12 and 14 by establishing the relationship R1<R2, where R1 and R2 are the resistances of the resistors R1 and R2 of the plasma generator 7.
The process by which a film is formed on the substrate is next described in somewhat further detail. The plasma 22 created in the vacuum chamber 1 by the plasma generator 7 gives energy to the particles evaporated from the evaporable material 2 and the oxygen gas from the process gas inlet port 20. Some of them are excited and ionized. As a result, the high-density plasma 22 is created inside the vacuum chamber 1. Furthermore, electrons are accumulated on the surface of the substrate dome 5 exposed to the plasma 22. A negative voltage is applied to the surface of the substrate dome.
Meanwhile, the plasma 22 has a zero or positive potential and so there is a difference in potential between the plasma 22 and the substrate dome 5 near the surface of the dome 5. The ions within the plasma 22 near the substrate dome are accelerated toward the substrate, thus bombarding it.
The bombardment is combined with the excitation and ionization of the evaporated particles and process gas to permit the quality of the film formed on the substrate to be improved. That is, the packing density of the film is enhanced, and the adhesion is improved.
Where an optical thin film is formed by a thin-film deposition system, the film is strongly required to have optical characteristics which do not change with environmental variations. For this purpose, it is important to enhance the packing density of the film. In the above-described thin-film deposition system, bombardment of ions present close to the substrate dome 5 against the substrate greatly contributes to the packing density.
Accordingly, in the aforementioned thin-film deposition system, the difference between the negative potential at the substrate dome 5 and the positive potential possessed by the plasma 22 is increased by increasing the density of the plasma 22. This increases the energy with which the ions present close to the dome 5 are accelerated toward the substrate. As a result, the packing density of the film can be enhanced. An optical thin film having improved environmental resistance can be formed.
However, if the density of the plasma 22 within the vacuum chamber 1 is enhanced, the temperature of the substrate dome 5 exposed to the plasma is elevated greatly with the elapse of time. There is the danger that the maximum processing temperature of the substrate will be exceeded.
If coating is done at a temperature lower than the maximum processing temperature of the substrate by lowering the density of the plasma 22, ions accelerated toward the substrate have lower energies, and the packing density of the film is not enhanced. There is the problem that the quality of the film is deteriorated.
It is desirable to be capable of modifying the energy of the ions bombarded against the substrate at will according to the kind of the evaporable material. In the present system, however, it is impossible to control the density of the plasma 22 within the vacuum chamber and the energy of the ions accelerated toward the substrate independently. Hence, it is not possible to finely establish the conditions under which thin films are formed.